The invention relates to a continuous steel casting method for a casting capacity of more than 1.5 metric tons of steel per minute. The method uses a vertically arranged, substantially rectangular mold, into which at least one steel stream containing inert gas is passed through a casting tube below the surface of the casting level in the mold. The casting tube is provided with lateral outlet openings directed downward, whose axes lie in a vertical plane running through the bigger transverse axis of the mold.
It is known in continuous steel casting to provide a gas-permeable, ring-shaped disk of refractory material below the bottom opening of the tundish. Through this disk inert gas, preferably argon, is blown radially in the direction of the casting stream axis. Gas bubbles are carried downward together with the steel stream, then rise from the liquid phase in the continuous casting mold together with nonmetallic particles contained in the steel and wash these particles into a slag layer floating on the casting level in the mold (CONCAST NEWS, Vol. 10, 1/1971, p.4 and FIG. 11). The gas bubbles also cause a decrease of the penetration depth of the metal stream in the mold, which diminishes the danger of crack formation. The use of refractory, gas-permeable disks or other intermediate pieces has, however, the disadvantage that the gas has to be supplied under pressure. The control of the gas supply is also problematic and operationally unreliable due to the danger that the gas channels might become blocked. Moreover, a gas veil forms between the casting tube and the steel stream. Also it is not possible to supply the gas in such a way that its amount is distributed evenly over the entire stream cross section. It has therefore been proposed to provide in the tundish an outlet consisting of two parts, the outer part being formed by a highly gas-permeable material and the inner part by a less gas-permeable material, so that the inert gas can be carried both in the vertical direction upward, and radially and vertically in relation to the steel stream (German Utility Model No. 7.149.261). Course non-metallic inclusions are to be carried upward into the slag layer of the tundish before they enter the outlet, while the remaining gas quantity is carried downward through the casting tube, in order to brake or slow the casting stream. The construction of this known outlet is complicated, its production is expensive and its operational reliability is not sufficient. There is no guarantee that the gas supplied can be distributed evenly over the entire metal stream cross section. The same applies analogously to outlet stones i.e. refractory bricks that form part of the stopper for a ladle and have a passage for steel flow, having a lateral bore for the supply of a gas (German Utility Model No. 6,918,019).
In continuous steel casting it is essential that the strand be free from surface and inner cracks, since defects of this kind lead to material losses, because cracked strands have to be scarfed or even rejected. Since modern continuous casting plants are to work fully automatically, a further demand is that the safety precautions against breakthroughs of the molten steel through the solidified strand skin, be increased. The breakthroughs that hitherto occurred repeatedly in continuous casting plants led to severe damages of the plant and the production loss was considerable. However, practice has shown that these problems are relatively insignificant up to a casting capacity of 1.5 metric tons of steel per minute. If, however, one increases the casting capacity to more than 1.5 metric tons of steel per minute, it becomes apparent that the hitherto known technology is no longer sufficient. In rapid casting plants for slabs having a thickness of 150 to 250 mm and a width of 800 to 2500 mm and more, the crack formation and the possibility of breakthroughs increases rapidly, as the casting velocity rises, because the steel flow causes cavitations in the range of the solidified strand skin, which are all the more critical, the thinner the strand skin. As is known, the thickness of the strand skin decreases, as the casting capacity increases. The steel flow in the liquid phase of the strand is substantially determined by the direction of the axes of the lateral openings of the casting tube. Normally refractory casting tubes having a closed bottom and two lateral openings inclined downward and directed toward the narrow sides of the mold are used. When the casting velocity is high, mainly edge cracks and breakthroughs in the range of the strand edges occur. If one casts with a casting stream directed exclusively vertically downward, longitudinal cracks at the broad side of the slabs may occur in that range, where the flow is strongest. Moreover, non-metallic inclusions are carried to a great depth in the liquid phase, since they have no opportunity to rise and to get into the slag floating on the surface of the molten steel (casting level). If in a casting tube having a closed bottom, the lateral outlets are directed steeply or perpendicularly upward, at a high casting velocity the casting level in the mold is agitated too strongly, which renders the slag practice in the mold more difficult.